1. Field of the Invention
The present invention relates to a drive device of a light emitting element in which a current drive type light emitting element is employed, and particularly to a drive device constructed in such a way that the light emitting element is individually driven to be lit by a combination of a constant current source and a current regulating resistor and a drive method thereof.
2. Description of the Related Art
Demand for a display panel which has a high definition image display function and a thin shape and which can realize low power consumption is increasing due to the spread of portable telephones, portable digital assistants (PDAs), and the like, and conventionally a liquid display panel has been adopted in many products as a display panel which meets requirements therefor. Meanwhile, recently, an organic EL (electroluminescent) element which makes the best use of a property of being a self light emitting type display element is also adopted in the above-mentioned some products, and this has attracted attention as a next generation display panel which replaces a conventional liquid display panel. This is because of backgrounds one of which is that by employing, in the light emitting layer of the organic EL element, an organic compound which enables an excellent light emission characteristic to be expected, a high efficiency and a long life which can be equal to practical use have been advanced.
The organic EL element is constructed basically in such a way that a transparent electrode for example by ITO, an organic EL medium, and a metallic electrode are laminated one by one on a transparent substrate. The organic EL medium may be a single layer of an organic light emitting layer, a medium of double layer structure composed of an organic positive hole transport layer and an organic light emitting layer, a medium of triple layer structure composed of an organic positive hole transport layer, an organic light emitting layer, and an organic electron transport layer, or a medium of multilayer structure in which an injection layer of electron or positive hole is inserted into an appropriate portion among these layers.
The above-described organic EL element can be electrically replaced by a structure composed of a light emitting component having a diode characteristic and a parasitic capacitance component which is connected in parallel to this light emitting component, and thus the organic EL element can be said to be a capacitive light emitting element. When a light emission drive voltage is applied to this organic EL element, at first, electrical charges corresponding to the electric capacity of this element flow into the electrode as a displacement current and are accumulated. It can be considered that when the drive voltage then exceeds a determined voltage (light emission threshold voltage=Vth) peculiar to this element, current begins to flow from one electrode (anode electrode side of the diode component) to an organic layer constituting the light emitting layer so that the element emits light at an intensity proportional to this current.
As a display panel in which such organic EL elements (hereinafter simply referred to also as EL elements) are employed, a panel of a dot matrix structure in which EL elements having approximately the same light emitting area are arranged respectively at intersection positions between drive lines and scan lines has been provided, and by this panel, various characters, images, and the like can be expressed through lighting patterns of dots. Meanwhile, in the above-mentioned portable telephones, PDAs, and the like, it is necessary for specific display patterns such as remaining power of a battery, a reception signal intensity, other incoming call/message display to be displayed on a part of the display panel.
These specific patterns are displayed utilizing icon patterns formed by allowing a transparent electrode by ITO to correspond to the display patterns, without utilizing the above-mentioned dot matrix. In the case where such icon patterns are formed, resolution can be improved more than that of a display by the dot matrix. The structure of the display panel in which a dot matrix area and an icon pattern area are formed on one panel surface while utilizing the organic EL elements as display elements is disclosed in Japanese Patent Application Laid-Open No. 2002-43053 that the present applicant has already filed.
FIG. 1 schematically shows an example of a display panel in which the icon patterns are formed and a function of a driver which supplies light emission drive power to these respective icon patterns. Reference numeral 1 shows a display panel, and in this display panel 1, for example, icon patterns displaying remaining power of a battery, a reception signal intensity, and the like are formed. These icon patterns are formed by allowing a transparent electrode by ITO to correspond to the display patterns of icons as described above.
That is, in a left side in FIG. 1, icon patterns showing the remaining power of a battery composed of four islands are formed, and in a state shown in FIG. 1, as shown by hatching the icon patterns, a state in which the remaining power of the battery is at second level is shown. In a right side thereof, it is shown that an icon pattern showing an antenna is in a lighting state, and in a further right side thereof, icon patterns including lines whose lengths in the vertical direction are different from one another are arranged. These show icon patterns showing a reception signal intensity, and the state shown in FIG. 1 shows a state in which the reception signal intensity which is shown by three levels is maximum.
Light emission drive power is supplied to the respective icon patterns via data lines (shown by thin lines in the drawing) formed by the same ITO on the display panel 1. Common scan lines (shown by dashed lines in the drawing) are arranged on the display panel 1, and back surface electrodes (the above-mentioned metallic electrode) of the respective icon patterns are mutually connected via these common scan lines.
Reference numeral 2 denotes a flexible printed circuit (FPC) in which respective lead lines are connected to the data lines and the common scan lines for example via an anisotropic conducting film (ACF) on an end portion of the display panel 1, and the other end portion of this flexible circuit substrate 2 is connected to a driver IC shown by reference numeral 3. The driver IC 3 is divided into a data side driver 3a and a scan side driver 3b, and the data side driver 3a is constructed such that necessary currents can be supplied in response to areas of the icon patterns on the display panel 1, respectively.
That is, a unit area for the icon patterns is considerably large compared to a dot pattern arranged in a matrix pattern, and the areas thereof considerably differ in accordance with the patterns. Further, there is a case where emission colors of organic EL elements constituting icons differ from one another, and thus light emission efficiencies thereof differ from one another, whereby large differences occur in the values of drive currents necessary for driving and lighting the respective icons.
FIG. 2 shows an example in which due to the above-described actual conditions, where one constant current source is a unit, two or three constant current sources are combined as the need arises so that current values necessary for driving and lighting the respective icons are obtained. In FIG. 2, reference numeral 1 shows the display panel as shown in accordance with FIG. 1, and E1-E11 show icon patterns one by one arranged on this display panel 1. For example, E1, E2, E3, . . . suppose the respective icon patterns showing the remaining power of the battery shown in FIG. 1, E5 supposes the icon pattern showing the shape of the antenna, and further E11 supposes the icon pattern of a horned moon figure.
Respective drive currents are supplied from the constant current sources to these respective icons E1-E11 via respective drive switches Sa1-Sa11 in the data side driver 3a and a scan switch Sk1 in the scan side driver 3b. In this case, to the respective icons E1, E2, E3, . . . which show the remaining power of the battery, respective drive currents are independently supplied from the respective constant current sources I1, I2, I3, . . . Since the light emitting area of the icon E5 showing the shape of an antenna is large, current from two constant current sources I5, I6 is supplied. Further, since the light emitting area of the icon E11 of the horned moon figure is the largest, current from three constant current sources I16-I18 is supplied.
In this case, although for example the icons E1, E2 showing for example the remaining power of the battery have a rectangular shape and the same area together, since their icon light emitting colors are different, a measure is taken where for example supply time of drive current for example in one frame period is controlled to substantially make the light emission intensities even. Further, similarly, a measure is taken where supply times of drive currents are appropriately controlled in response to area ratios of respective icons. Such a manner of performing time gradation is schematically shown by the lengths of white portions showing the respective constant current sources in the data side driver 3a in FIG. 1. In other words, hatched portions show non-supply time (current shutting off period) of drive current from the respective constant current sources.
As described above, in order to drive and light the icon patterns respectively at appropriate intensities at a good balance, it is necessary to calculate the number of constant current sources by a unit of one constant current source as described above, preferably a unit of 0.5 constant current sources, and also to regulate current supply times (lighting time rates) supplied from the constant current sources. Further, in the case where multilevel dimmer control is to be realized by lighting times for all icons, lighting times of the respective icons have to be divided into smaller pieces, resulting in extremely complex control.
Such an appropriate allotment for the constant current sources is normally performed by setting registers in the driver IC or the like. When products in which light emitting modules are loaded are different, it also is normal that icon patterns are different, and a custom (exclusive use) driver IC is needed for each product. Therefore, preparing a custom IC as a driver for each light emitting module directly causes an increase in cost.
Meanwhile, regarding the organic EL element, it has been known that the light emission lifetime of the EL element can be prolonged by sequentially applying a reverse voltage (reverse bias voltage) which does not contribute to a light emission operation, and this is disclosed for example in Japanese Patent Application Laid-Open No. Hei 11-8064 and the like.